Ion exchange methods for removing biuret from urea

ABSTRACT

Methods are provided for removing biuret from biuret-containing aqueous urea solutions by contacting the biuret-containing urea solutions with the hydroxide ion form of an anion exchanger under anion exchange conditions in which methods one or more aqueous process streams are either formed from materials which are substantially or completely free of alkaline earth metal and/or carbonate impurities or which process streams are treated either before or during their use in the anion exchange process to reduce their content of contaminant alkaline earth metal and/or carbonate compounds. The anion exchanger can be washed and regenerated with one or more aqueous media including water, aqueous solutions of strong base, acidic chloride solutions, and/or other aqueous solutions, and either batch or continuous (fixed bed) contacting can be employed. The use of one or more aqueous process streams in such methods which have been prepared from aqueous media of relatively low alkaline earth metal and/or carbonate content or which are treated either before or during their use in the methods of this invention, markedly reduces the degree and rate of anion exchanger deactivation and the severity of regeneration required to restore anion exchanger activity and thereby markedly increases anion exchanger efficiency and useful life.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of urea purification, and inparticular, it relates to methods for removing biuret from urea.

2. Description of the Art

Urea is a widely used fertilizer and chemical precursor. Most often itcontains some biuret that forms during the urea manufacturing process orwhen urea is otherwise heated to 130° C. or above. Biuret can interferewith chemical processing and is toxic to many plants. Its phytotoxicityhas been thoroughly studied, and it is regulated and monitored bygovernment agencies and industry. For instance, the Indian governmentprohibits the import of urea containing more than 2 weight percentbiuret. The United States agricultural industry generally observes anupper limit of 0.25 weight percent biuret for urea fertilizersclassified as "low biuret." This criterion is generally recognized bythe citrus and other industries that use urea for foliar fertilization.

Detectable biuret toxicity symptoms have been noted in field tests onlemon and grapefruit in Southern California at biuret levels as low as0.1 weight percent. Biuret toxicity has also been observed withtopically applied urea prills and solutions. Seed germination inhibitionand damage to seedlings has been observed in wheat, barley and similargrain crops at levels of 2 weight percent biuret.

Damage to corn has been observed at foliar biuret dosages of 0.2 to 0.5kilogram per hectare. Thirty percent yield loss was noted in one studyat 1.7 kilograms biuret per hectare banded near seeds. Wheat damage hasbeen observed at 0.2 to 0.5 kilogram per hectare foliarly applied, andsevere toxicity was observed at 6.0 kilograms per hectare biuret bandedin the soil. Fifteen to twenty ppm soil biuret level has been shown toinhibit barley seed germination while substantial crop damage fromfoliar application often occurs at 0.4 to 0.6 kilogram biuret perhectare.

Similar effects have been observed in rice, citrus, cotton, avocado,beans, soybeans and potatoes, several of which are particularlysensitive to biuret in foliar fertilizers. In citrus, as little as 0.2kilogram foliarly applied biuret per hectare causes detectable damage.Avocados are damaged by as little as 50 ppm biuret in foliar sprays. Aslittle as 3 kilograms per hectare biuret banded in the soil inhibitspotato germination and causes citrus damage in light soils. Thesestudies, and a comprehensive review of the literature available on thissubject, are presented by Mithyantha, Kulkarni, Tripathi andAgnihothrudu, Fertilizer News, 1977, pp. 13-18.

In view of these results, it is not surprising that the industry hasdevoted substantial effort to methods of preventing biuret formation inthe first instance, and to methods of reducing its concentration once itis formed. Most contemporary commercial urea plants are capable ofproducing solid and solution urea containing much less biuret than waspreviously the case. However, essentially all commercial ureas containat least 0.5 weight percent biuret, and most contain from 1 to 2 weightpercent biuret. Biuret content can rise considerably higher ifmanufacturing conditions are not adequately controlled.

One method for removing biuret from urea solutions is described byFuentes et al. in U.S. Pat. No. 3,903,158, issued Sept. 2, 1975. Fuenteset al. describe a procedure in which a urea solution containing biuretis passed over either anionic or cationic exchange resins which,according to Fuentes et al., selectively retain biuret and allow urea topass through the resin. The exchange resin can then be regenerated bycontact with a basic solution after which the resin can be reused.

Another process for removing biuret from urea, disclosed by Young andGreen in U.S. Pat. No. 4,345,099, involves treating a biuret-containingurea solution at a pH of about 12.5 or higher and a temperature of about0° C. to about 100° C. under which conditions the biuret is hydrolyzedand thereby eliminated from the solution.

Takahashi and Yoshida, in "Determination of Biuret in Urea by IonExchange Resins", Soil and Plant Food, Volume 3, No. 3 January 1958,pages 142-144, disclose a process similar to that described by Fuenteset al., supra, in which biuret is removed from aqueous urea solutions bycontact with a basic anion exchange resin. According to Takahashi etal., the biuret is quantitatively retained on the resin, even afterwater washing, thus allowing quantitative determination of biuret inaqueous urea solutions. The resin can be regenerated by acidicsolutions, such as hydrochloric acid solutions, which contain chlorideion.

Another procedure for removing biuret from urea which is sufficientlyquantitative to allow for its use as an analytical procedure, isdisclosed by Geurts, Steele and Brinkman in "Determination of Biuret inUrea Mixed Fertilizers," Analytica Chimica Acta, Volume 41, (1968) atpages 113 through 120. Geurts et al. disclose that biuret, which isfirst complexed with copper while in solution with urea, can bequantitatively removed from the solution by contact with certain ionexchange resins, and that the copper-biuret complex is not displacedfrom the resin by 0.9 molar ammonia or 0.3 molar sodium hydroxidesolutions but can be eluted with 2 molar potassium nitrate followed by0.2 molar nitric acid extraction.

General references to the characteristics and utility of anionexchangers such as strongly basic anion exchange resins are found in thetrade literature such as Rohm & Haas Product Bulletin "Amberlite"IRA-400, Bulletin IE-16-56, revised April 1956, which discloses thatAmberlite IRA-400 is a strongly basic anion exchange resin which canextract negative ions from either acidic, neutral or basic solutions.Biuret is known to be negatively charged in aqueous solutions. Thus, theRohm & Haas bulletin suggests that strongly basic anion exchange resinssuch as Amberlite IRA-400 are capable of removing negatively chargedions such as biuret from either acidic, neutral or basic solutions.

Against this background, it can be seen that biuret can be effectivelyremoved from aqueous urea solutions by the use of basic anion exchangeresins, and that such methods require the use of relatively expensiveanion exchanger and regeneration techniques.

Strongly basic anion exchangers such as Amberlite IRA-400 cost in therange of about $50 to about $150 per cubic foot. The strongly caustic oracidic solutions required to regenerate the exchangers are alsorelatively expensive. These regenerants must be sufficiently strong todislodge the biuret from anion exchanger. Since, according to theliterature, the biuret is relatively strongly held by the anionexchanger (a feature which would be beneficial from the standpoint ofassuring adequate removal of biuret from the urea solution), the artsuggests that relatively severe regeneration conditions are required toefficiently remove the biuret from the deactivated anion exchanger.Obviously, the cost of anion exchanger regeneration, the cost ofconstructing, maintaining and operating a system capable of removingbiuret from a certain quantity of urea solution, and the expense of theanion exchanger required in the process, all increase as the frequencyand/or severity of regeneration increases. Thus, the requirement forfrequent and/or more severe regeneration increases the regenerant costsand the amount of anion exchanger and the size of the operating facilityrequired to treat a given amount of urea solution.

We have now found that the frequency and severity of regeneration, thequantity of required anion exchanger, and the size of the operatingplant required to remove biuret from aqueous urea solutions can besignificantly reduced by employing novel biuret exchange and exchangerregeneration procedures which increase the efficiency and rate withwhich biuret is removed from a biuret-containing urea feed solution andwhich reduce anion exchanger deactivation.

It is therefore one object of this invention to provide improved methodsfor removing biuret from aqueous urea solutions.

Another object is the provision of methods for removing biuret from ureasolutions by ion exchange which reduce the rate and severity of anionexchanger deactivation.

Another object is the provision of methods for removing biuret from ureaby ion exchange which increase the useful life of the anion exchanger.

Yet another object is the provision of methods for removing biuret fromurea by ion exchange in which the frequency and duration of exchangerregeneration are reduced.

Yet another object of this invention is to reduce the cost of regenerantdisposal associated with the removal of biuret from urea by ion exchangein which the ion exchanger is regenerated with basic regenerant.

Yet another object of this invention is to increase the rate ofproduction of low biuret urea from biuret-containing solutions.

Yet another object is the provision of methods for the removal of biuretfrom urea by ion exchange which reduce the exposure of the ion exchangerto deactivating components.

Other objects, aspects and advantages of this invention will be apparentto one skilled in the art in view of the following disclosure, thedrawing, and the appended claims.

SUMMARY OF THE INVENTION

The methods of this invention involve the removal of biuret frombiuret-containing aqueous urea solutions by contacting thebiuret-containing urea solutions with the hydroxide form of an anionexchanger under conditions sufficient to remove at least a portion ofthe biuret from the biuret-containing urea solution and retain theremoved biuret on the anion exchanger, in which methods one or more ofthe aqueous process streams which contact the anion exchanger are either(A) treated, before, after or during their use in the process, to removealkaline earth metal compounds and/or carbonate anion or (B) areprepared from aqueous media of low alkaline earth metal and/or carbonatecontent. The principal process streams which contact the anion exchangerinclude the aqueous biuret-containing urea feed solution and otheraqueous media which are employed either to regenerate the anionexchanger by removing biuret abstracted from the urea feed or to washthe anion exchanger to remove residual feed solution, regenerant,accumulated solid debris, or to backwash and reclassify the anionexchanger particles employed in the preferred fixed bed anion exchangeembodiment.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be more readily understood by reference to thedrawing which is a schematic illustration of a cyclical ion exchangesystem which can be employed in accordance with one embodiment of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides methods for removing biuret from urea bycontacting a biuret-containing urea solution with an anion exchangerunder conditions sufficient to remove at least a portion of the biuretfrom the urea feed solution and retain the removed biuret on the anionexchanger in which methods the calcium equivalent and/or carbonateconcentrations in one or more of the aqueous solutions with which theanion exchanger is contacted during the process are controlled atrelatively low levels. The term "calcium equivalent content" isconventionally employed to connote the concentration of "hard water"components which are primarily alkaline earth metal compounds and isemployed in the description of this invention in its conventional sense.

The principal aqueous media with which the anion exchanger is contactedduring the process include the biuret-containing aqueous urea feedsolution and other aqueous media which may optionally include aqueouswash media which are usually employed to remove residual processsolutions and/or extraneous solid matter from the anion exchanger, andaqueous regenerants. The aqueous regenerants useful in these methodsinclude aqueous solutions of strong base, relatively non-alkalineaqueous regenerants, and chloride containing aqueous solutions, whichare employed primarily to remove biuret and other contaminants such asalkaline earth metal and carbonate deposits from the anion exchanger. Inaccordance with the methods of this invention, one or more of suchsolutions, and preferably all solutions with which the anion exchangeris contacted during the process, (A) contain less than about 1 calciummilliequivalent per liter, (B) have been treated to reduce their calciumequivalent content, (C) have been prepared from water which containsless than about 1 calcium milliequivalent per liter, and/or (D) havebeen prepared from water which has been treated to reduce its calciumequivalent content. Optionally, one or more, and preferably all, of suchaqueous process streams, (A) contain about 35 parts per millioncarbonate anion or less, (B) have been treated to reduce their carbonatecontent, (C) are prepared from water which contains about 35 parts permillion carbonate anion or less, and/or (D) have been prepared fromwater which has been treated to reduce its carbonate anion content.Preferably, the methods of this invention involve the control of boththe calcium equivalent content and the carbonate anion content of one ormore, and preferably all, of the aqueous process streams which contactthe anion exchanger.

The methods of this invention reduce or eliminate the expense andinefficiencies associated with several aspects of processes which areotherwise available for removing biuret from urea. For instance, theuseful life and efficiency of the anion exchanger are both increased, inaccordance with the methods of this invention, in part, by eliminatingexchanger-deactivating components from the urea feed and/or otheraqueous process streams. Increased exchanger efficiency increases theamount of biuret that can be removed from the urea feed solution in eachcycle and reduces regeneration cost. Increased exchanger useful liferesults in obvious economy and further reduces process down time whichwould otherwise be required to recharge the system with fresh anionexchanger.

These benefits result in higher overall production rates of low or zerobiuret urea and reduce the overall process expense required to produce agiven quantity of low biuret urea product. Further economies result fromreduced caustic regenerant disposal expense since much less regenerantis discharged in the methods of this invention than is the case inalternative processes such as those disclosed by Fuentes et al.

The biuret-containing, aqueous urea feed solutions which can be treatedin accordance with the methods of this invention include all suchsolutions which are either manufactured or employed in any industry.Most of these solutions contain at least about 0.5 weight percent,generally about 1 to about 90 weight percent and most commonly about 1to about 70 weight percent urea. Greater economy is realized in thesemethods by employing relatively concentrated urea solutions in order toreduce the volume of material handled and reduce the exposure of theanion exchanger to impurities contained in the urea feed solution. Thus,preferred urea solutions usually contain about 10 to about 70 weightpercent urea.

The biuret concentration in the urea feed can vary considerably but willusually be at least about 0.1 weight percent biuret based on urea. Mostcommercial ureas, however, contain at least about 0.5 weight percent,generally about 0.1 to about 10 weight percent, and most commonly about0.1 to about 5 weight percent biuret based on urea.

Anion exchangers which are useful in this invention can be eitherorganic or inorganic, basic anion exchangers or combinations of organicand inorganic anion exchangers. The anion exchanger is preferably atleast moderately basic and is most preferably a strongly basic anionexchanger such as the anion exchangers marketed by Rohm & Haas Companyunder the trademark Amberlite® IRA-400, IRA-458, and IRA-900, and thelike, anion exchangers marketed by the Dow Chemical Company under thetrademark Dowex I-X4, anion exchangers marketed by the BIORad Companyunder the trademark AG MP-1, and others.

The presently preferred anion exchangers are the strongly basic, organicion exchange resins which contain tertiary and/or quaternary aminegroups. Such anion exchangers can be prepared by the chloromethylationof a styrenedivinylbenzene copolymer which is then reacted with atertiary or secondary amine; by the condensation of phenylenediaminewith formaldehyde; or by the condensation of phenylenediamine,polyethyleneimine and formaldehyde. Particularly preferred anionexchangers are the quaternary amine types such as those disclosed inU.S. Pat. No. 2,591,573, the disclosure of which is incorporated hereinby reference.

The hydroxide ion form of the anion exchangers is presently preferred toeffect the removal of biuret from the biuret-containing urea solutionsin accordance with the methods of this invention. However, many of thecommercially available anion exchangers are manufactured and sold inother ionic forms, such as the chloride form, and require conversion tothe hydroxide ion form prior to use in the methods of this inventionConversion of the anion exchanger to the preferred hydroxide ion formcan be readily accomplished by contacting the anion exchanger with anaqueous solution of a strong base as described, for instance, in "IonExchange with the Amberlite Resins" and "Amberlite IRA-400- LaboratoryManual", both of which are available from the Resinous ProductsDivision, Rohm & Haas Company, Washington Square, Philadelphia, 5,Pennsylvania, and both of which are incorporated herein by reference.

The aqueous solutions of strong base which are useful optionalregenerants in the methods of this invention can be employed to convertthe non-hydroxide forms of anion exchangers, e.g., the chloride form, tothe preferred hydroxide form and to regenerate the biuret-containinganion exchanger after the exchanger has been employed to remove biuretfrom the urea feed solution The strong bases useful for these purposesinclude both organic and inorganic bases and should be capable ofproducing a pH in water of at least about 11.5, preferably at leastabout 13, and most preferably about 14. Illustrative organic andinorganic bases include tetramethylammonium hydroxide, triethylamine,ammonium hydroxide, and the alkali hydroxides and their water solubleprecursors such as lithium, sodium, potassium, cesium and rubidiumhydroxides and hydroxide precursors. The alkali metal hydroxides,particularly sodium and potassium hydroxides and hydroxide precursors,are presently preferred. Although ammonium hydroxide is a useful basicregenerant in concentrations up to about 10 weight percent ammonia,surprisingly, it interferes with rather than benefits regeneration atsubstantially higher ammonia concentrations, e.g., 25 weight percentammonia, particularly with the preferred organic anion exchanger resins.

The concentration of base in the strong base solutions useful in themethods of this invention will usually be at least about 0.5 weightpercent, normally about 0.5 to about 25 weight percent, preferably about1 to about 10 weight percent and most preferably about 2 to about 6weight percent. The base concentration should be sufficient to convertthe anion exchanger to the desired hydroxide ion form and/or to displacebiuret retained on the anion exchanger after the anion exchanger hasbeen employed to remove biuret from the urea feed solution.

Chloride ion-containing solutions can be employed in accordance with themethods of this invention to regenerate the deactivated anion exchangerwhen more severe regeneration is desired than can be obtained readilywith the strong base. For instance, it is sometimes useful to employchloride ion-containing solutions to remove excessive accumulations ofcarbonate and/or alkaline earth compound deposits and/or otherimpurities which accumulate on or in the anion exchanger. The chlorideion-containing solutions useful for this purpose are aqueous solutionsof ionizable organic or inorganic chloride-containing compounds such asthe alkali metal chlorides which usually contain at least about 0.5,preferably at least about 1, and generally about 1 to about 10 weightpercent of the chloride-containing compound. Sodium and potassiumchlorides are presently preferred. Acidic chloride ion-containingsolutions more rapidly and effectively regenerate the anion exchangersthan do non-acidified solutions. Preferred acidified solutions normallyhave pH levels of about 3 or less. Relatively dilute hydrochloric acidsolutions and sodium chloride solutions which have been acidified withan acid, such as hydrochloric acid, are presently preferred.Hydrochloric acid, when employed, will usually be present in the acidicchloride solution at a concentration of at least about 0.001 normal,preferably at least about 0.01 normal and generally about 0.1 to about 3normal.

Water is conventionally employed to backwash the ion exchange resin toremove accumulated debris, to expand and reclassify the bed of anionexchanger particles, and/or to remove residual process fluids such asthe process feed or regenerant, either prior to regeneration or prior toreuse following regeneration. However, we have found that water andother non-alkaline aqueous media can also be employed to remove asignificant proportion of the biuret retained on the anion exchangerafter its use. The discovery that anion exchangers which efficientlyremove biuret from aqueous solutions can be regenerated by anon-alkaline aqueous media such as pure water was surprising, and itsuse as an optional regenerant can result in substantial economies in themethods of this invention. Water or other non-alkaline aqueous media canbe employed either as a partial regenerant prior to further regenerationwith the aqueous solution of a strong base, or it can be used as theonly regenerant in one or more cycles. Such use of non-alkalineregenerants markedly reduces the amount of strong base which is requiredto maintain anion exchanger activity.

The substantially non-alkaline aqueous media employed as regenerantsherein have pH levels below about 11.5, preferably below about 10, mostpreferably below about 8 and will usually contain less than 0.1 weightpercent of a strong base. The term "substantially non-alkaline" isemployed herein to distinguish aqueous regenerants and wash media whichhave pH levels below 11.5 from the more strongly alkaline regerantswhich are aqueous solutions of a strong base and which have pH levels of11.5 or above. The substantially non-alkaline aqueous media can containother components which are not strongly basic and which do not diminishthe ability of the non-alkaline media to remove buiret from the anionexchanger. Thus, the non alkaline aqueous media can contain minoramounts of weak base, chloride ion, and/or other components.

At least one of the aqueous process streams should be treated to reduceits calcium equivalent (alkaline earth metal) content and/or carbonatecontent, and/or should be prepared from water of low calcium equivalentand/or carbonate content or from water which has been treated to reduceits calcium equivalent and/or carbonate content. Preferably, the calciumequivalent and/or carbonate contents of all of the aqueous processstreams employed in these methods are controlled by such procedures.

We have found that the activity and efficiency of the anion exchangerscan be markedly diminished by contaminants present in the urea feedsolution, the water from which all of the aqueous solutions employed inthe process are derived, and which are generated in the process itself,particularly in the strong base recycle system. Principal among thesecontaminants are alkaline earth metal compounds and carbonate ion.Carbonate ion (including carbonate compounds) is usually present incommercially available urea solids (prills, granules, etc.) and ureasolutions, and may be present in the water employed in any one of theaqueous streams employed in the methods of this invention. We have alsofound that carbonate ion is generated in the aqueous solution of astrong base following its use to remove biuret from the anion exchanger.The so-called "hard" water components, principally calcium and magnesiumions, also contribute to anion exchanger deactivation and inefficiency.The greater portion, if not all, of these hard water components enterthe system in the water employed to produce one or more of the aqueousprocess streams.

As a result of these findings, we have discovered that the activity,efficiency, and lifetime of the anion exchanger can be markedlyimproved, and that the amount of strong base, non-alkaline, and chlorideion regenerants required to maintain anion exchanger activity andefficiency can be markedly reduced, by employing aqueous process streamswhich have relatively low concentrations of carbonate and/or "hard"water contaminants.

It is presently preferred that the "hardness" of the water employed inone or more of the aqueous process streams in the methods of thisinvention correspond to less than 1, preferably less than 0.5, and mostpreferably less than about 0.2 calcium milliequivalents per liter. Theselimits correspond approximately to less than about 20 parts per million,preferably less than 10 parts per million, and most preferably less thanabout 4 parts per million equivalent calcium ion. Preferably, theaqueous solutions themselves, including the biuret-containing urea feed,the aqueous solution of a strong base, the non-alkaline regenerant, washmedia, and other aqueous process fluids, also meet this criteria.

The terms "equivalent calcium" and "calcium milliequivalents per liter"are conventionally used in the art to refer to the degree of water"hardness" and include water hardness that is attributable primarily toalkaline earth metals, especially calcium and magnesium. The value forcalcium milliequivalents per liter can be determined bycomplexometric-colorometric titration with ethylenediamine tetraaceticacid (EDTA) as discussed by Homer D. Chapman and P. F. Pratt in "Methodsof Analysis for Soils, Plants and Waters," University of California,Division of Agricultural Sciences, 1961. The approximate concentrationof equivalent calcium (which includes calcium and other alkaline earths)in parts per million can be determined by multiplying the value forcalcium milliequivalents per liter by 20.

Calcium equivalent content can be reduced and/or controlled by the useof naturally occurring soft waters such as some mountain streams andrivers and by the use of distilled and ion exchanged water sources.Suitable exchanged waters include waters which have been decationized(hydrogen exchanged), deionized, sodium exchanged ("soft"), and watersources which have been exchanged with cations which do not form waterinsoluble carbonates.

The carbonate content (including bicarbonate which is often referred toas "temporary hardness") of the water employed to form the aqueousprocess streams useful in these methods, as well as the carbonatecontent of the process streams themselves, is advantageously maintainedat a level of about 35 parts per million or less, preferably about 20parts per million or less, and most preferably within the range of 0 to10 parts per million by weight.

Carbonate content can be controlled by the use of naturally occurringlow carbonate waters such as mountain streams and rivers, and by the useof distilled or ion exchanged water sources or combinations of suchsources. Suitable ion exchanged low carbonate water sources includewaters in which the carbonate anion has been replaced by one or moreanions which minimize competition with biuret for exchange sites.Various ions are suitable for this purpose with hydroxide ion being mostpreferred and nitrate, sulfate, and chloride ions being less preferred.The carbonate content of water supplies which are unacceptably high incarbonate content also can be controlled by removal of carbonatecontaminant by any one of several known procedures such as byprecipitation of the carbonate as insoluble alkaline earth metalcarbonate. Such carbonate precipitation can be achieved by adding to thewater supply an amount of an organic or inorganic alkaline earth metalcompound and separating the precipitated alkaline earth carbonate byfiltration, decanting, or other method of liquid-solid separation.Calcium and magnesium compounds, and combinations of such compounds, arepresently preferred. Illustrative alkaline earth metal compounds are thecalcium and magnesium hydroxides, chlorides, sulfates, nitrates,hydrogen phosphates, acetates, and the like, with the hydroxides beingparticularly preferred.

The alkaline earth metal compound is preferably added only in the amountnecessary to react with and precipitate the carbonate contained in thewater supply or process stream. Thus, the amount of added alkaline earthmetal compound preferably does not exceed the stoichiometric amountrequired to react with carbonate. Most preferably, the alkaline earthmetal compound is added in an amount corresponding to about 90 percentor less of the stoichiometric amount of alkaline earth metal required toreact with the carbonate in order to reduce the possibility ofcontaminating the anion exchanger with alkaline earth metals.

The described water and process stream selection and/or treatmentprocedures are usually sufficient to reduce the concentration ofalkaline earth metals and/or carbonate to acceptable levels. Forinstance, sodium exchange can reduce equivalent calcium content to lessthan 1 part per million (less than 0.05 calcium milliequivalents perliter), and deionization can be employed to reduce calcium content tolevels of about 0.5 parts per million (0.025 calcium milliequivalentsper liter).

In accordance with this invention, at least a portion of the biuretcontained in a biuret-containing aqueous urea solution is removed bycontacting the urea solution with the hydroxide ion form of an anionexchanger useful in this invention under conditions sufficient to removeat least a portion of the biuret from the urea solution and retain thethus removed biuret on the anion exchanger. The urea solution of reducedbiuret content is recovered from the anion exchanger, and the anionexchanger is contacted with fresh quantities of the biuret-containingurea solution until either the desired quantity of reduced biuret ureaproduct has been obtained or the capacity of the anion exchanger forremoving biuret from the urea solution has diminished to a point thatregeneration is required. At that point, the ion exchanger can beoptionally washed an regenerated in accordance with this invention bycontacting the anion exchanger with an aqueous regenerant underconditions which are sufficient to remove at least a portion of thebiuret retained on the anion exchanger and restore at least a portion ofthe activity of the anion exchanger for removing biuret from additionalquantities of the biuret-containing urea solution. The thus formedbiuret-containing regenerant is then recovered from the anion exchangerand the cycle can be repeated as desired to remove biuret from freshquantities of the biuret-containing urea solution.

The methods of this invention can be employed to reduce the biuretcontent of the biuret-containing urea solutions to essentially anydesired level depending on the treatment conditions chosen. Normally,however, the process conditions will be selected to produce a producturea solution having a biuret content of about 0.5 weight percent orless, often of about 0.2 weight percent or less, and, if desired, ofabout 0.1 weight percent or less. In fact, the methods of this inventioncan be employed to reduce the biuret content of any selectedbiuret-containing aqueous urea solution to undetectable levels.

All of the process steps, including the extraction of biuret from thebiuret-containing urea solution and the anion exchanger washing andregeneration steps, can be performed either by batch contacting or bythe more efficient continuous plug flow contacting in which the ureafeed and regenerant solutions are passed through the anion exchangerwhich is retained in a relatively fixed bed. Plug flow systems can beoperated either downflow or upflow, although downflow systems aregenerally preferred.

Each increment of the biuret-containing urea solution is usuallycontacted with the anion exchanger for a period of at least about 30seconds, preferably at least about one minute, most preferably at leastabout 5 minutes, and generally about one minute to about one hour.Contact times of about 5 to about 30 minutes are usually adequate toeffect the desired degree of biuret removal from the urea feed solution.Such contact times correspond to flow rates of about 2 bed volumes perminute or less, preferably about 1 bed volume per minute or less, mostpreferably about 0.2 bed volumes per minute or less, and usually about0.02 to about 1 bed volume per minute.

Contact of the anion exchanger with the biuret-containing feed isusually continued until the desired quantity of low biuret urea solutionhas been prepared or until the capacity of the anion exchanger isdepleted. Depletion of exchanger capacity is indicated in the preferredcontinuous, fixed bed systems by biuret breakthrough which occurs when adetectable quantity of biuret is present in the urea solution productrecovered from the anion exchanger. However, when greater quantities ofbiuret can be tolerated in the urea product, the biuret extraction stepcan be continued past the point of biuret breakthrough. The level ofbiuret acceptable in the urea solution product can be set at anyacceptable level, e.g., 60 ppm, 0.5 weight percent, etc.

After completion of the biuret extraction step the urea solutionremaining in contact with the anion exchanger can be recovered from theanion exchanger and either returned to the urea feed solution reservoir,to the product accumulator, or otherwise as desired.

It is sometimes desirable, although not essential, to backwash the resinto remove foreign matter, to flush remaining urea solution from theanion exchanger, and/or to "reclassify" the bed of anion exchangerparticles. Backwashing is usually effected by passing water rapidlyupwardly through the bed to expand the bed by, e.g., 50 percent, afterwhich the anion exchanger is regenerated.

The anion exchanger then can be regenerated by contact under ionexchange conditions with an aqueous regenerant such as an aqueoussolution of a strong base or a substantially non-alkaline aqueousmedium, e.g., water or other aqueous media. Both a non-alkaline mediumand an aqueous solution of a strong base can be employed sequentiallyand preferably in that order.

Regeneration of the anion exchanger with water or other non-alkalineaqueous media is conducted under ion exchange conditions anddistinguishes from water washing which usually involves much lowervolumes of water which are passed through the anion exchanger at a muchmore rapid rate than is involved in ion exchange. Indeed, we have foundthat when a non-alkaline aqueous medium such as water is slowly passedover the anion exchanger containing retained biuret under conditions ofcontact time and regenerant volume suitable for ion exchange, thenon-alkaline aqueous medium removes a substantial proportion of theretained biuret from the anion exchanger.

Regeneration of the anion exchanger is achieved by contacting the anionexchanger with a sufficient quantity of the aqueous regenerant for aperiod of time sufficient to remove a substantial proportion of theretained biuret from the anion exchanger. The contact time andregenerant volume required to achieve the desired degree of regenerationdepends, in part, on the strength of the regenerant, the basicity of theanion exchanger, the quantity of biuret and/or other contaminants whichare to be removed from the anion exchanger, and, to a lesser extent, onthe method of contacting employed--batch or continuous. Longer contacttimes and/or larger regenerant volumes are generally required when usingweaker (less basic) regenerants, stronger (more basic) anion exchangers,and/or batch contacting. Surpisingly, however, we have found that waterand other non-alkaline aqueous media, although not basic in the usualsense of that term, are very effective for the removal of biuret fromeven very strong anion exchangers. Indeed, water is as effective forthis purpose as dilute ammonium hydroxide (10 weight percent ammonia)and is more effective than concentrated ammonium hydroxide (25 weightammonia percent). Continuous, plug flow (preferably downflow)regeneration is generally much more efficient than is batch mixing andis therefore preferred.

A substantial proportion of the retained biuret usually can be removedby contacting the anion exchanger with a sufficient quantity of theaqueous regenerant for a period of at least about 20 minutes, preferablyat least about 30 minutes, and generally about 30 minutes to about 10hours. Regenerant volume is conveniently expressed in terms of thevolume of anion exchanger to be regenerated (bed volumes in fixed bedsystems) and will usually correspond to at least about 5, preferably atleast about 8, most preferably at least about 12, and generally about 5to about 100 volumes of regenerant per volume of anion exchanger. Therate at which the regenerant is passed over the anion exchanger, in thepreferred plug flow technique, is conveniently expressed in terms of bedvolumes of regenerant per unit time and usually corresponds to about 3bed volumes per minute or less, preferably about 1 bed volume per minuteor less, most preferably about 0.5 bed volumes per minute or less, andgenerally about 0.05 to about 1 bed volumes per minute.

Batch regeneration is usually accomplished by contacting thebiuret-containing anion exchanger with a portion, e.g., 5 to 50 percentof the total regenerant volume to be employed for a period of timesufficient to allow the anion exchanger and regenerant to reachequilibrium. The contact time required for the anion exchanger to reachequilibrium with an increment of regenerant in batch regeneration isusually less than that required when continuous, plug flow regenerationis employed and generally corresponds to about 5 to about 50 percent ofthe contact time discussed above with respect to continuousregeneration. However, since batch regeneration usually requires anumber of contacting steps each of which employs a separate increment ofregenerant, the total contact time required for batch regeneration isusually the same as or greater than that required in the preferred plugflow system.

Regeneration is usually continued until the activity of the anionexchanger has been sufficiently restored to allow its efficient reusefor the removal of biuret from additional quantities of thebiuret-containing urea feed solution. It is generally desirable tocontinue regeneration until at least about 80 percent, and preferably atleast about 90 percent, of the initial biuret exchange capacity of theanion exchanger has been restored. The extent of regeneration can beconveniently determined by monitoring the biuret content in theregenerant effluent from the anion exchanger during regeneration.Regeneration can be terminated when the biuret content of the effluenthas diminished to some acceptable level such as 200 parts per million orless. More complete regeneration is evidenced by lower biuretconcentrations in the regenerant effluent.

After regeneration is complete, the anion exchanger optionally can bewashed to remove residual regenerant and can be employed to removebiuret from additional quantities of the urea feed solution.

In a particularly preferred embodiment of this invention the aqueousregenerant employed in at least one cycle of the process is an aqueoussolution of a strong base which has been recovered from the anionexchanger in a previous regeneration step.

In a particularly preferred embodiment of this invention, the biuretremoved from the biuret-containing aqueous urea solution and the anionexchanger is periodically regenerated in a multi-cycle process in whichmethod, in one or more cycles, the anion exchanger regenerant comprisesan aqueous solution of a strong base which has been employed toregenerate the anion exchanger in a previous cycle. This procedureallows for recycling and reuse of the strong base regenerant. Thisprocedure results in substantial savings primarily to the reduced costof strong base regenerants required to regenerate the anion exchangerand to reduce caustic disposal expense. Thus, this embodiment of theinvention enables the reuse of the strong base regenerant which resultsin substantial economy in the overall process. The aqueous solution of astrong base can be reused for at least one cycle, usually at least twocycles, preferably for at least four cycles, and generally for aboutfour to about twenty cycles before it is discharged from the system. Wehave found that it is generally desirable to recycle the aqueoussolution of a strong base until the base has been approximately halfconsumed by regeneration of the anion exchanger.

The number of cycles in which an aqueous solution of a strong base canbe employed depends on the initial quantity or inventory of strong basesolution relative to the quantity of biuret to be removed, the initialconcentration of strong base in the basic regenerant solution, theamount of biuret that is retained on the anion exchanger in each cycle,and the presence of impurities in the system such as alkaline earthmetal ions and carbonate ion. For instance, one gallon of strong baseregenerant having an initial sodium hydroxide concentration of about 4weight percent will be approximately half depleted in regenerating onegallon of an anion exchanger which has been employed to remove 0.5weight percent biuret from 9 gallons of an aqueous urea solution. Onegallon of basic regenerant having an initial sodium hydroxideconcentration of 6 weight percent could be employed to regenerate morethan one gallon of the same anion exchanger under otherwise identicalconditions. Similarly, nine gallons of the basic regenerant whichinitially contains 4 weight percent sodium hydroxide can generally beemployed to regenerate one gallon of the depleted anion exchangerapproximately nine times. Thus, nine volumes of the basic regenerantcould be employed to regenerate one volume of the anion exchanger innine separate cycles in accordance with this embodiment of theinvention.

The several aspects, objects and advantages of the methods of thisinvention can be better understood by reference to the drawing, whichschematically illustrates one embodiment of an anion exchange systemuseful in one embodiment of this invention and which includes ionexchange vessel 1, biuret-containing urea feed solution reservoir 7,acidic chloride regenerant reservoir 5, basic regenerant feed reservoir6, recycle reservoir accumulator 9 and the more essential process flowlines. As illustrated in the drawing, biuret-containing urea feedsolution is passed from reservoir 7 via lines 11, 12 and 13 throughdistributor 4 and into contact with the hydroxide ion form of a basicanion exchanger retained in fixed bed 2. Urea solution of reduced biuretcontent is removed from vessel 1 via line 14 and 15. Followingcompletion of the biuret extraction step of the cycle, the anionexchanger bed 2 can be backwashed by municipal water which enters thesystem via line 19 and is passed to the lower portion of vessel 1 vialines 20 and 14. Overflow from the backwashing step exits vessel 1 vialine 30 and can be passed to waste. In the alternative, before anionexchanger 2 is backwashed with water, residual urea solution can bewashed downwardly through the bed by passing water into the vessel 1 vialines 19, 18, 13 and distributor 4 and can be removed from vessel 1 vialines 14 and 15 and returned to feed storage vessel 7 by conduits notillustrated in the drawing.

In accordance with this invention, water of reduced carbonate and/orcalcium equivalent content can be supplied to the process from vessel 8which can be a treated water reservoir or an ion exchange vesselcontaining one or more ion exchangers which are capable of reducing thecarbonate and/or calcium equivalent content of water entering the vesselvia line 16. Treated water can be passed from vessel 8 via lines 17, 18and 13 through distributor 4 to downwash the anion exchanger bed 2 andoptionally can be passed upwardly through the anion exchanger via lines17, 20, and 14. Treated water from vessel 8 can also be directed vialine 31 to supply water of reduced calcium equivalent and/or carbonatecontent to vessels 5, 6 and 7, via lines 33, 34, and 32, respectively,for formation of the desired urea feed, acidic chloride, and aqueousbase solutions.

After completion of the exchanger washing step, the anion exchanger canbe regenerated with an aqueous regenerant which can be an aqueoussolution of a strong base, a substantially non-alkaline aqueous medium,or a combination of the substantially non-alkaline medium and the strongbase employed sequentially and preferably in that order as describedherein. When water is employed as the regenerant, it can comprise waterfrom a local source (line 19) and preferably comprises treated waterfrom vessel 8, either of which is preferably passed over the anionexchanger downflow via lines 18 and 13 through distributor 4. Spentaqueous regenerant containing biuret removed from the anion exchanger,is recovered from anion exchange vessel 1 via line 14 and can bedischarged from the system via line 15.

As an alternative to regeneration with the substantially non-alkalineaqueous medium described immediately above, or as a supplement to suchregeneration, the anion exchanger can be regenerated by contact with anaqueous solution of a strong base useful in the methods of thisinvention which can be passed from reservoir 6 via lines 23, 21, 12 and13 through distributor 4 into downflow contact with anion exchanger 2.Basic regenerant solution is recovered from anion exchanger 2 via lines14 and 24 and can be returned to reservoir 6 via lines 25 and 26. Thisstep of the cycle can be continued until the anion exchanger has beencontacted with the desired amount of the aqueous solution of a strongbase.

In the alternative, the anion exchange column 1 can be charged with arelatively small quantity, e.g., as little as about 1 to about 2 bedvolumes of aqueous base solution from reservoir 6, and the aqueous baseeffluent from the anion exchanger can be recycled directly into contactwith the anion exchanger via lines 14, 36, 21, 12 and 13. For instance,if it is desired to regenerate the anion exchanger with 6 bed volumes ofthe aqueous solution of a strong base, a lesser quantity of the basicsolution, e.g., 2 bed volumes, can be charged to anion exchange vessel 1as described above and recycled three times via lines 36, 21, 12 and 13.By this procedures, 2 bed volumes of the aqueous basic regenerant can beemployed to contact the anion exchanger with the equivalent of 6 bedvolumes of regenerant during a single regeneration step.

In another embodiment the methods of this invention provide for theelimination of at least a portion of the biuret from the recycledaqueous solution of a strong base. We have discovered that biuret isgradually eliminated from the recycled base solution by hydrolysis bythe strong base, and that biuret elimination can be accelerated byheating the recycled base to a temperature of at least about 40° C.,generally within the range of about 40° to about 100° C., for at leastabout 30 minutes Such heat treatment of the recycled basic regeneratedin the conveniently accomplished in the embodiment illustrated in thedrawing by passing regenerant from ion exchange vessel 1 via lines 14,24 and 27 to vessel 9 provided with heating element 35, filter 10 andvent 28. Treated regenerant is returned to vessel 6 via lines 29 and 26.

Optionally, vessel 9 can also be employed to remove carbonate from therecycled basic regenerant. Carbonate removal can be accomplished bymixing a water soluble metal compound with the basic regenerant invessel 9 which compound reacts with carbonate to form insolublecarbonate as described herein. Contamination of recycled basicregenerant by metal carbonate precipitate can be prevented by suitablefilter means 10 in vessel 9.

Although the regeneration procedures discussed above are generallysufficient to maintain acceptable activity of the anion exchanger for anumber of cycles, e.g., 5 cycles or more, alkaline earth and/orcarbonate contamination of the anion exchanger may gradually accumulateto the point that exchanger activity and/or capacity becomesunacceptably low. The occurrence of low exchange activity or capacity isevidenced, in part, by relatively rapid biuret breakthrough into the ionexchanger effluent during the biuret extraction step and indicates theneed for more severe regeneration. Such regeneration can be effected bycontacting the anion exchanger with a chloride solution which, in thesystem illustrated in the drawing, is passed from reservoir 5 via lines22, 21, 12, and 13 through distributor 4 into contact with anionexchanger 2. Contacting of the anion exchanger with the chloridesolution should be continued until a substantial proportion, generallyat least about 80 percent, and preferably at least about 90 percent ofthe anion exchanger capacity has been converted to the chloride form.This degree of chloride exchange is generally sufficient to remove amajor proportion of accumulated impurities from the anion exchanger, andit can be effectively accomplished by contacting the anion exchangerwith the chloride solution for a period of at least about 2 minutes,generally at least about 5 minutes, and preferably about 5 minutes toabout 2 hours. These conditions correspond to the use of at least about1 bed volume, preferably at least about 2 bed volumes, and generallyabout 3 to about 60 bed volumes of the chloride solution passeddownwardly over the anion exchanger at a rate of less than about 3 bedvolumes per minute, preferably less than about 1 bed volume per minute,and most preferably less than 0.5 bed volumes per minute in thecontinuous fixed bed system illustrated in the drawing.

When the anion exchanger contains substantial amounts of carbonate,carbon dioxide will be generated when the anion exchanger is contactedwith an acidic solution. Therefore, if the chloride solution employed inthis step is acidified, provision is preferably made for allowing thecarbon dioxide to disengage from the anion exchanger and from thechloride solution. This can be achieved by passing the solution over theanion exchanger in increments or by batch contacting the anion exchangerwith the solution outside the column.

Regeneration procedures particularly suitable for any particular anionexchanger are usually available from the manufacturer. However, theregeneration conditions discussed immediately above are generallysuitable for most basic anion exchangers which are useful in the methodsof this invention.

After completion of the chloride exchange step, the anion exchanger canbe reconverted to the active hydroxide ion form by washing the anionexchanger to remove residual acidic chloride solution and regeneratingit with the aqueous solution of a strong base as described herein.

Numerous variations and modifications of the methods of this inventionas illustrated in the drawing will be apparent to one skilled in the artand are intended to be encompassed within the appended claims. Forinstance, two or more ion exchange vessels 1 can be employed and can beoperated alternately in two or more steps of the ion exchange cycle. Forexample, one ion exchanger can be employed to remove biuret from thebiuret-containing urea feed solution while another ion exchange vesselcan be undergoing regeneration. Similarly, the biuret decomposition andcarbonate removal functions which can be performed in vessel 9 also canbe performed in separate vessels or in the aqueous base regenerantreservoir 6.

The anion exchanger can also be regenerated or otherwise separated fromaccumulated deposits and/or impurities by removing it from anionexchange vessel 1 and treating the anion exchanger, ex situ of thesystem, sequentially with chloride solution, concentrated aqueoussolution of a strong base, e.g., 25 weight percent sodium hydroxide, orother regeneration medium which may be recommended by the manufactureror which is known in the art. The regenerated anion exchanger then canbe re-packed in the anion exchange vessel for reuse. Indeed, all stepsof the process can be conducted by batch processing if desired.

The invention is further described by the following examples which areillustrative of specific modes of practicing the invention and are notintended as limiting the scope of the invention as defined by theappended claims.

EXAMPLE 1

This example demonstrates that the use of process water and aqueous feedand regenerant solutions which have been treated to reduce their calciumequivalent and carbonate content enables the use of a basic anionexchanger for 17 cycles of biuret extraction and strong baseregeneration even though the basic regenerant was not replenished andwas recycled in each cycle.

A glass column is packed to a height of 60 cm. with 280 ml. of thechloride form of the anion exchanger marketed by Rohm & Haas Companyunder the trademark Amberlite IRA-458. The anion exchanger was convertedto the active hydroxide form by contact with 4 weight percent aqueoussodium hydroxide solution which is formed from deionized water (free ofcarbonate ion and calcium equivalents) by passing approximately 10 bedvolumes (2.8 liters) of the sodium hydroxide solution downwardly overthe anion exchanger bed at a rate of approximately 0.2 bed volumes perminute. The hydroxide exchanged anion exchanger is then washed withdeionized water passed downflow over the exchanger to remove residualsodium hydroxide. The sodium hydroxide-containing chloride ion removedfrom the anion exchanger is discharged from the system.

The thus activated anion exchanger is then employed to remove biuretfrom an aqueous solution containing 45 weight percent urea and about 1weight percent biuret which is formed by dissolving biuret-containingurea in deionized water. The urea solution is passed downwardly over theanion exchanger at a rate of about 0.2 bed volumes per minute untilbiuret breakthrough is observed at which time the anion exchanger hasremoved 14.3 grams of biuret from the urea solution feed. Thiscorresponds to a biuret capacity of approximately 51 grams biuret perliter of anion exchanger.

The anion exchanger is then drained to remove residual urea solution andis regenerated by contact with 4 weight percent sodium hydroxidesolution formed from deionized water which is passed downwardly over theanion exchanger at a rate of approximately 0.2 bed volumes per minute.

After each regeneration cycle the sodium hydroxide regenerant isrecovered and returned to the regenerant reservoir which initiallycontains 10 bed volumes (2.8 liters) of 4 weight percent sodiumhydroxide regenerant which is not replenished during the operation.

The biuret extraction and sodium hydroxide regeneration steps arerepeated 16 times for a total of 17 cycles. Sodium hydroxide regenerantvolume employed in each cycle varies from 5.4 to 21 bed volumes percycle with the average being about 9 bed volumes per cycle which issufficient to regain the initial biuret removal capacity of the anionexchanger through 15 cycles of operation. Exchanger capacity forremoving biuret from the urea feed solution diminishes in the 16th and17th cycles indicating the accumulation of impurities on the anionexchanger and substantial reduction of the regenerative ability of thesodium hydroxide regenerant.

After the 17th cycle the anion exchanger is regenerated withapproximately 10 bed volumes of fresh 4 weight percent sodium hydroxidesolution formed from deionized water which is passed downwardly over theanion exchanger at a rate of about 0.2 bed volumes per minute. Thisregeneration is sufficient to restore the activity of the anionexchanger for the removal of biuret from the urea feed solution to alevel of about 50 grams of biuret per liter of packed anion exchangerbed which corresponds to the initial activity of the anion exchanger.

EXAMPLE 2

This example illustrates that the use of urea feed solutions, regenerantsolutions, and process water which have been treated to reduce carbonateand calcium equivalent content, markedly improves anion exchangeractivity and useful life for the removal of biuret frombiuret-containing aqueous solutions.

The operation described in Example 1 is repeated by passing thebiuret-containing aqueous urea solution described in Example 1downwardly over the anion exchanger employed in Example 1 until biuretbreakthrough is observed. The anion exchanger is then drained of ureafeed solution, regenerated with 4 weight percent sodium hydroxidesolution formed from deionized water, and water-rinsed to removeresidual sodium hydroxide solution as described in Example 1 with theexception that the rinse water employed is obtained from the municipalwater supply and contains about 300 ppm total dissolved solids and about150 ppm equivalent calcium carbonate. This calcium content correspondsto about 70 ppm equivalent calcium or 3.5 calcium milliequivalents perliter. This operation is repeated three additional times for a total offour cycles. The municipal water rinse volume is approximately 10.5 bedvolumes per cycle and the sodium hydroxide regenerant (10 bed volumestotal sodium hydroxide solution) is recovered and reused in eachsuccessive cycle.

After the completion of the fourth cycle the activity of the anionexchanger for the removal of biuret from the urea feed solution has beendiminished by 50 percent to an equivalent of about 25 grams of biuretper liter of anion exchanger.

EXAMPLE 3

This operation demonstrates that a basic anion exchanger containingretained biuret which has been removed from an aqueous urea solution canbe regenerated by contact with water which has been treated to reduceits calcium equivalent content and carbonate content.

The ion exchange column described in Example 1 is charged to a packedvolume of 200 ml with the chloride form of Amberlite IRA-458 which isconverted to the hydroxide form as described in Example 1. The resultinghydroxide form of the anion exchanger is then employed to remove biuretfrom an aqueous urea solution containing 45 weight percent urea and 0.44weight percent biuret which was formed by dissolving urea containingapproximately 1 weight percent biuret in deionized water. The ureasolution is passed downwardly over the anion exchanger at a rate ofabout 0.2 bed volumes per minute until biuret is detected in the ureasolution effluent from the anion exchanger at a detection limit of 60ppm biuret. At this point, 2,040 grams of the aqueous urea solution havebeen passed over the anion exchanger and 9.2 grams of biuret have beenretained on the anion exchanger, representing complete recovery ofbiuret from the urea feed solution.

The anion exchanger is then drained to remove residual urea feedsolution and is contacted with 20 bed volumes of deionized water whichis passed downwardly over the anion exchanger at a rate of 0.2 bedvolumes per minute. This treatment is sufficient to remove 7.0 grams ofbiuret from the anion exchanger which corresponds to 76 percent biuretrecovery.

EXAMPLE 4

The operation described in Example 3 is repeated with the exception thatanion exchanger Amberlite IRA-900 is substituted for Amberlite IRA-458.All other conditions are the same as described in Example 3.

Biuret is first detected in the urea solution effluent the anionexchanger after 2,000 grams of urea solution have been passed over theanion exchanger. This corresponds to a biuret retention of 8.8 grams ofbiuret on the anion exchanger.

The Amberlite IRA-900 is regenerated by contacting it with 20 bedvolumes of deionized water passed downwardly over the resin at a rate of0.2 bed volumes per minute. This regeneration removes 7.8 grams ofbiuret from the anion exchanger representing a biuret recovery of 88.3percent.

EXAMPLE 5

The operation described in Example 3 is repeated with the exception thatanion exchanger Amberlite IRA-400 is substituted for Amberlite IRA-458.All other conditions including feedstocks and regenerants are the sameas described in Example 3.

Biuret is first detected in the urea solution effluent from the anionexchanger after 2,340 grams of urea feed solution have been passedthrough the anion exchange column at which time 10.3 grams of biurethave been retained on anion exchanger. The biuret-containing anionexchanger is regenerated with deionized water as described in Example 3to recover 7.9 grams of biuret which represents a biuret recoveryefficiency of 76.7 percent.

EXAMPLE 6

Amberlite IRA-400 anion exchanger is packed into a tubular glass ionexchange column in the manner described in Example 1 and is converted tothe hydroxide form by exchange with 4 weight percent sodium hydroxidesolution (formed from deionized water). The anion exchanger is thenwashed with deionized water to remove residual sodium hydroxide solutionas described in Example 1. The packed exchanger bed containsapproximately 200 ml. of the exchanger and has a finished bed height ofabout 35 cm. The anion exchanger is then contacted with aqueous ureasolution containing 50 weight percent urea and 0.6 weight percent biuret(1.2 weight percent biuret based on urea) passed downflow through theanion exchange column at a rate of 0.1 bed volumes per minute untilbiuret is first detected in urea solution effluent from the anionexchange column. The column is then drained to remove residual ureasolution and is backwashed with deionized water to physically reclassifythe resin beads according to size and to remove particulate matter fromthe anion exchanger bed.

Biuret is first observed in the urea solution effluent from the anionexchange column after the column has removed the equivalent of 0.57milliequivalents of biuret per ml. of anion exchanger.

The anion exchanger is then regenerated by contacting with 32.5 bedvolumes of deionized water passed downwardly through the anion exchangercolumn at a flow rate of 0.07 bed volumes per minute.

The regenerated anion exchanger is then employed to remove biuret fromadditional quantities of the urea feed solution under the conditionsemployed in the first cycle of this operation. Urea solution flow iscontinued until biuret is detected in the urea solution effluent fromthe anion exchange column which occurs after the anion exchanger hasremoved the equivalent of 0.48 milliequivalents of biuret per ml. ofanion exchanger.

The anion exchanger is backwashed and regenerated with deionized water asecond time under the conditions employed in the first regeneration stepwith the exception that 37.5 bed volumes of deionized water regenerantare employed in this step.

The anion exchanger is employed to remove biuret from a third quantityof the urea feed solution until biuret is detected in the urea solutioneffluent which occurs after the anion exchanger has removed theequivalent of 0.45 milliequivalents of biuret per ml. of resin.

These results demonstrate that regeneration with water which has beentreated to reduce its equivalent calcium and/or carbonate content isadequate to enable the repeated use of anion exchangers for the removalof biuret from biuret-containing urea solutions.

Numerous variations and modifications of the concepts of this inventionwill be apparent to one skilled in art in view of the aforegoingdisclosure, the drawing, and the appended claims, and are intended to beencompassed within the scope of this invention defined by the followingclaims.

We claim:
 1. A method for removing biuret from a biuret-containing,aqueous urea solution which method comprises the steps of(A) contactingsaid biuret-containing urea solution with the hydroxide ion form of ananion exchanger under conditions sufficient to remove at least a portionof said biuret from said biuret-containing urea solution and retain thebiuret thus removed on said anion exchanger, (B) recovering the thusformed urea solution of reduced biuret content from said anionexchanger, and (C) contacting said anion exchanger containing said thusremoved biuret with at least one other aqueous medium selected from thegroup consisting of aqueous wash media, aqueous regenerants, andcombinations thereof, wherein at least one of said biuret-containingurea solution and said other aqueous medium contains less than about 1calcium milliequivalent per liter.
 2. The method defined in claim 1wherein said other aqueous medium comprises at least one aqueousregenerant, and said anion exchanger is contacted with said aqueousregenerant under conditions sufficient to remove at least a portion ofsaid retained biuret from said anion exchanger.
 3. The method defined inclaim 1 wherein at least one of said biuret-containing urea solution,said aqueous wash media, and said regenerant has been treated to reduceits calcium equivalent content.
 4. The method defined in claim 1 whereinat least one of said biuret-containing urea solution, said wash media,and said regenerant contains less than about 0.5 calciummilliequivalents per liter.
 5. The method defined in claim 1 wherein atleast one of said biuret-containing urea solution, said wash media, andsaid regenerant has been formed from water which has been treated by amethod selected from the group consisting of deionization, hydrogenexchange, sodium-exchange, distillation, and combinations thereof. 6.The method defined in claim 1 wherein at least one of saidbiuret-containing urea solution, said wash media, and said regeneranthas been formed from water which has been ion exchanged to reduce itscalcium equivalent content.
 7. The method defined in claim 1 wherein atleast one of said biuret-containing urea solution, said wash media, andsaid regenerant has been formed from water which contains less thanabout 0.5 calcium milliequivalents per liter.
 8. The method defined inclaim 1 wherein at least one of said biuret-containing urea solution,said wash media, and said regenerant has been formed from water whichcontains less than about 0.2 calcium milliequivalents per liter.
 9. Themethod defined in claim 1 wherein said other aqueous medium compriseswater which has been treated to reduce its calcium equivalent content.10. The method defined in claim 1 wherein said biuret-containing ureasolution and said other aqueous medium comprise water which has beentreated to reduce its calcium equivalent content.
 11. The method definedin claim 2 wherein said aqueous regenerant comprises an aqueous solutionof a strong base.
 12. The method defined in claim 1 wherein said otheraqueous medium comprises an aqueous wash medium, and saidbiuret-containing urea solution and said aqueous wash medium compriseaqueous media which have been treated to reduce their calcium equivalentcontent.
 13. The method defined in claim 1 wherein said other aqueousmedium comprises an aqueous wash medium, and said aqueous wash mediumand said biuret-containing urea solution contain less than about 0.5calcium milliequivalents per liter.
 14. The method defined in claim 1,wherein said other aqueous medium comprises an aqueous wash medium, andsaid aqueous wash medium and said biuret-containing urea solution havebeen formed from water which has been treated to reduce its calciumequivalent content.
 15. The method defined in claim 1 wherein said otheraqueous medium comprises an aqueous wash medium, and said aqueous washmedium and said biuret-containing urea solution have been formed fromwater which has been ion-exchanged to reduce its calcium equivalentcontent.
 16. The method defined in claim 1 wherein said other aqueousmedium comprises an aqueous wash medium, and said aqueous wash mediumand said biuret-containing urea solution have been formed from waterwhich contains less than about 0.2 calcium milliequivalents per liter.17. The method defined in claim 1 wherein said other aqueous wash mediumand said biuret-containing urea solution have been formed from waterwhich contains less than about 0.1 calcium milliequivalents per liter.18. The method defined in claim 1 wherein said other aqueous mediumcomprises an aqueous wash medium and an aqueous regenerant, and saidbiuret-containing urea solution, said aqueous wash medium and saidregenerant each comprise less than about 0.5 calcium milliequivalentsper liter.
 19. The method defined in claim 18 wherein said aqueous washand said aqueous regenerant are each selected from the group consistingof water, aqueous solutions, and combinations thereof.
 20. The methoddefined in claim 18 wherein said aqueous regenerant comprises an aqueoussolution of a strong base.
 21. The method defined in claim 18 whereinsaid aqueous wash medium, and said regenerant each have been formed fromwater which has been treated by a method selected from the groupconsisting of deionization, hydrogen exchange, sodium exchange,distillation, and combinations thereof.
 22. The method defined in claim18 wherein said biuret-containing urea solution, said aqueous washmedium, and said regenerant have been formed from water which has beenion-exchanged to reduce its calcium equivalent content.
 23. The methoddefined in claim 18 wherein said biuret-containing urea solution, saidaqueous wash medium, and said regenerant have been treated to reducetheir calcium equivalent content.
 24. The method defined in claim 18wherein at least one of said biuret-containing urea solution, saidaqueous wash medium, and said regenerant contains less than about 0.2calcium milliequivalents per liter.
 25. The method defined in claim 1which comprises the step of contacting said anion exchanger, after saidrecovery of said urea solution of reduced biuret content, with asubstantially non-alkaline aqueous medium, wherein said substantiallynon-alkaline aqueous medium has been treated to reduce its calciumequivalent content.
 26. The method defined in claim 25 wherein saidanion exchanger is contacted with said substantially non-alkalineaqueous medium under conditions sufficent to at least wash from saidanion exchanger a substantial proportion of said biuret-containing ureasolution which remains on said anion exchanger.
 27. The method definedin claim 25 which further comprises the step of contacting said anionexchanger with an aqueous solution of a strong base under conditionssufficient to remove at least a portion of said retained biuret fromsaid anion exchanger.
 28. The method defined in claim 27 wherein saidaqueous solution of a strong base comprises water which has been treatedto reduce its calcium equivalent content.
 29. The method defined inclaim 1 wherein at least one of said biuret-containing urea solution,said aqueous wash, and said regenerant comprises an aqueous medium whichhas been treated to reduee its equivalent carbonate anion content. 30.The method defined in claim 1 wherein said other aqueous mediumcomprises an aqueous wash medium, and at least one of saidbiuret-containing urea solution and said aqueous wash comprises anaqueous medium which has been treated to reduce its equivalent carbonateanion content.
 31. The method defined in claim 30 wherein both saidbiuret-containing urea solution and said aqueous wash comprises anaqueous medium which has been treated to reduce its carbonate anioncontent.
 32. The method defined in claim 12 wherein at least one of saidbiuret-containing urea solution and said aqueous wash medium comprisesan aqueous medium which has been treated to reduce its equivalentcarbonate content.
 33. The method defined in claim 32 wherein both saidbiuret-containing urea solution and said aqueous wash medium comprise anaqueous medium which has been treated to reduce its equivalent carbonatecontent.
 34. A method for removing biuret from a biuret-containing,aqueous urea solution which method comprises the steps of(A) contactingsaid biuret-containing urea solution with the hydroxide ion form of ananion exchanger under conditions sufficient to remove at least a portionof said biuret from said biuret-containing urea solution and retain thebiuret thus removed on said anion exchanger, (B) recovering the thusformed urea solution of reduced biuret content from said anionexchanger, and (C) contacting said anion exchanger containing saidretained biuret with at least one substantially non-alkaline aqueousmedium, wherein said biuret-containing urea solution and saidsubstantially non-alkaline aqueous medium contain less than about 0.2calcium milliequivalents per liter.
 35. The method defined in claim 34which further comprises the step of contacting said anion exchanger,after said contacting with said substantially non-alkaline aqueousmedium, with an aqueous solution of a strong base under conditionssufficient to at least partially regenerate said anion exchanger. 36.The method defined in claim 34 wherein at least one of saidbiuret-containing urea solution and said substantially non-alkalineaqueous medium comprises an aqueous medium which has been treated toreduce its equivalent carbonate content.
 37. The method defined in claim36 wherein both said biuret-containing urea solution and saidsubstantially non-alkaline aqueous medium comprise an aqueous mediumwhich has been treated to reduce its equivalent carbonate content.
 38. Amethod for removing biuret from a biuret-containing, aqueous ureasolution which method comprises the steps of(A) contacting saidbiuret-containing urea solution with the hydroxide ion form of an anionexchanger under conditions sufficient to remove at least a portion ofsaid biuret from said biuret-containing urea solution and retain thebiuret thus removed on said anion exchanger, (B) recovering the thusformed urea solution of reduced biuret content from said anionexchanger, and (C) contacting said anion exchanger containing said thusremoved biuret with at least one other aqueous medium selected from thegroup consisting of aqueous wash media, aqueous regenerants, andcombinations thereof, whherein at least one of said biuret-containingurea solution and said other aqueous medium comprises an aqueous mediumwhich has been treated to reduce its calcium equivalent content.
 39. Amethod for removing biuret from a biuret-containing aqueous ureasolution which method comprises the steps of(A) contacting saidbiuret-containing urea solution with the hydroxide ion form of an anionexchanger under conditions sufficient to remove at least a portion ofsaid biuret from said biuret-containing urea solution and retain thebiuret thus removed on said anion exchanger, (B) recovering the thusformed urea solution of reduced biuret content from said anionexchanger, and (C) contacting said anion exchanger containing said thusremoved biuret with at least one other aqueous medium selected from thegroup cosisting of aqueous wash media, aqueous regenerants, andcombinations thereof, wherein at least one of said biuret-containg ureasolution and said other aqueous medium which contains less than about 1calcium milliequivalent per liter.
 40. A method for removing biuret froma bouret-containing, aqueous urea solution which method comprises thesteps of(A) contacting said biuret-containing urea solution with thehydroxide ion form of an anion exchanger under conditions sufficient toremove at least a portion of said biuret from said biuret-containingurea solution and retain the biuret thus removed on said anionexchanger, (B) recovering the thus formed urea solution of reducedbiuret content from said anion exchanger, and (C) contacting said anionexchanger containing said thus removed biuret with at least one otheraqueous medium selected from the group consisting of aqueous wash media,aqueous regenerants, and combinations thereof, wherein at least one ofsaid biuret-containing urea solution and said other aqueous medium isformed from water which ahs been treated to reduce its calciumequivalent content.
 41. A method for removing biuret from abiuret-containing, aqueous urea solution which method comprises thesteps of(A) contacting said biuret-containing urea solution withhydroxide ion form of an anion exchanger under conditions sufficient toremove at least a portion of said biuret from said biuret-containingurea solution and retain the biuret thus removed on said anionexchanger, (B) recovering the thus formed urea solution of reducedbiuret content from said anion exchanger containing said retained biuretwith at least one substantially non-alkaline aqueous medium, (C)contacting said anion exchanger containing said retained biuret with atleast one substantially non-alkaline aqueous medium, wherein saidbiuret-containing urea solution and said substantially non-alkalineaqueous medium each comprise an aqueous medium which has been treated toreduce its calcium equivalent content.
 42. A method for removing biuretfrom a biuret-containing, aqueous urea solution which method comprisesthe steps of(A) contacting said biuret-containing urea solution withhydroxide ion form of an anion exchanger under conditions sufficient toremove at least a portion of said biuret from said biuret-containingurea solution and retain the biuret thus removed on said anionexchanger, (B) recovering the thus formed urea solution of reducedbiuret content from said anion exchanger containing said retained biuretwith at least one substantially non-alkaline aqueous medium, (C)contacting said anion exchanger containing said retained biuret with atleast one substantially non-alkaline aqueous medium, wherein saidbiuret-containing urea solution and said substantially non-alkalineaqueous medium are each formed from water which contains less than about0.2 calcium milliequivalents per liter.
 43. A method for removing biuretfrom a biuret-containing, aqueous urea solution which method comprisesthe steps of(A) contacting said biuret-containing urea solution withhydroxide ion form of an anion exchanger under conditions sufficient toremove at least a portion of said biuret from said biuret-containingurea solution and retain the biuret thus removed on said anionexchanger. (B) recovering the thus formed urea solution of reducedbiuret content from said anion exchanger containing said retained biuretwith at least one substantially non-alkaline aqueous medium, (C)contacting said anion exchanger containing said retained biuret with atleast one substantially non-alkaline aqueous medium, wherein saidbiuret-containing urea solution and said substantially non-alkalineaqueous medium are each formed from water which has been treated toreduce its calcium equivalent content.